Due to the sharp cutoff capability and high cost performance, digital filters are increasingly used in many applications, instead of analog filters. digital filters are of two types, i.e., IIR (infinite impulse response) filter and FIR (finite impulse response) filter. For digital radio, FIR low-pass filters are employed because of their linear operating characteristic for input-output phase relationship. Since the shift registers that comprise a digital filter are the primary source of aliasing, or fold-over distortion noise, they are clocked at twice the symbol rate for oversampling the incoming signal. The clock rate of the shift registers thus determines the maximum operating speed of a digital filter. If higher transmission speed is desired for a system, the clock rate of the digital filters must be increased. However, the operating speed of a digital filter is limited to the maximum operating speed of the integrated circuit components that comprise the shift registers and other constituent parts of the filter.
One approach would be to design a digital filter with two parallel subsystems each being operated respectively on two clock sequences each being out of phase by .pi. radian with respect to the other. However, the hardware cost of the filter is twice as much as the current FIR digital filter.